If you are having trouble verifying your DKIM record, there are two useful tools you can use to help fix them. Keep in mind that DNS changes might take several hours to propagate.

Checking DKIM Records

Using
DKIM Core Key Check tool, insert the text before the"._domainkey" as the selector(it looks something like this: 20130425164621.pm) and then your domain. If DKIM is valid, it should show a result of This is a valid DKIM key record. If the record is not valid, go back to the instructions in Postmark and check the details to make sure your DNS records match.

Common DKIM Record Problems

If you have added your DKIM record, waited 48 hours, and you still aren’t seeing that DKIM is verified, check to make sure you can see the record in your DNS using a
dig command.

As an example, if my DKIM record should be in DNS at
20130425164621.pm._domainkey.postmarkapp.com, I can check to see if it is there using this command:

$ dig 20130425164621.pm._domainkey.postmarkapp.com txt

The dig command should return the DKIM record value seen in the authentication page. If it does not, it means the record is not in the correct location. Double check to ensure that you have added the record with the correct host/name in your DNS.

Sometimes DNS providers will automatically add your root domain to the location, so it could have been added at
xxxxxxxxxxxxxx.pm._domainkey.domain.com.domain.com instead of the intended location. In that case, add the record with the host/name as xxxxxxxxxxxxxx.pm._domainkey. instead of xxxxxxxxxxxxxx.pm._domainkey.domain.com.